Recent Posts

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91
Q & A / Re: 1970 1200s restoration
« Last post by Tdlunsfo on April 21, 2017, 05:10:08 am »
They are available but man they are expensive. $25 a piece. The ESR was pretty low on these at .64, almost half of my brand new can cap. I feel good leaving them in for now. Planning to dig into the trem today. Already suspecting this 22v zener and the 1 and 2uf caps. Not sure what's up with the bass. Could be tube related.
92
Q & A / Re: 1970 1200s restoration
« Last post by _peter on April 20, 2017, 10:58:43 pm »
Hi,

good to hear the amp is working again. Nevertheless, I would put in new electrolytic caps.
The  20 @ 600V might be hard to find. Here in Europe I got lucky on this weird page:
https://www.die-wuestens.de/eindex.htm
They have F&Ts with a 550/600V rating.

Cheers,
Peter
93
Classified / Re: WTB: Sunn Beta Lead Head
« Last post by vmattina on April 20, 2017, 09:02:56 pm »
I have a Sunn beta lead in excellent condition. I live in Salt Lake City so I'm not sure how I would get it to you.
94
Q & A / Re: 1970 1200s restoration
« Last post by Tdlunsfo on April 20, 2017, 06:27:39 pm »
Wired back in the old caps and then blew a fuse. Change fuse, fire it back up and notice a redplating 6550..uh oh! Quick check on the bias and I am somewhere in the -30v range instead of -55v. Re-biased the amp and the hum and slowly decreased. She sounds great. Tremolo is still a little weak but reverb sings. Amp lacks bass. Could be that the speaker is sitting out of the cab. Overall a very clean sounding amp. Threw a Big Muff Pi on it and she growled! This amp is a beast. If you can't cut through the mix with this thing then I don't know what you need. Lesson learned on the bias and swapping over from diode rectifier back to tubes.
95
Q & A / Re: 1970 1200s restoration
« Last post by Tdlunsfo on April 20, 2017, 12:53:53 pm »
You know what... your right. I have miss-counted the number of capacitors and didn't take into account that one of the 20uf on the cap can is being unused. I guess from working on these old tube radios, I was used to seeing these added after the fact and assumed the cap can took care of all the filtering. Thanks for helping me see the forest from the trees. This might even fix my tremolo issue as well. Will post back.
96
Q & A / Re: 1970 1200s restoration
« Last post by _peter on April 20, 2017, 10:49:33 am »
But those 20 @ 600V are the first two filter caps. You ripped them out and wonder why it's humming?

If I got you right, you have a huge ripple on the supply voltages now. That has less effect on the power amp,
as it works in push pull mode, but the ripple will modulate the preamp signal.

That's why the hum stops when you pull the PI.

I'll attach the schematic, so we know we are talking about the same thing. I guess you mean C29 and C30.

Cheers,
Peter
97
Q & A / Re: 1970 1200s restoration
« Last post by Tdlunsfo on April 20, 2017, 09:13:32 am »
Yes and I think so or at least one of them. The other appeared to be in the bias circuit. Can't find them on the schematic though. The filter cap was a 30,20,20,20 model but schematic shows a 40,30,20,20.
98
Q & A / Re: 1970 1200s restoration
« Last post by _peter on April 20, 2017, 09:05:31 am »
Hi,

you aren't talking about the 600V 20 filter caps, right?

Cheers,
Peter
99
Q & A / Sunn Coliseum Bass Prototype?
« Last post by landonverse on April 20, 2017, 08:56:34 am »
Grabbed this in a trade recently. Was told it was a 60's Coliseum Bass prototype. Serial etched into the back, 0101. Was also told there are only two known to exist, the other being in Ohio. Can any of you experts shed some insight on this amp.
100
Q & A / Model T problem
« Last post by Isaac on April 20, 2017, 07:40:03 am »
My Model T popped its circuit breaker at rehearsal last night. Reset it and it popped again. So something is wrong.

This amp hasn't had any issues at all for about 40 years. Back around 1975 or 76, the canned multicap was replaced at the Sunn factory. About ten years ago, I did a couple of mods to make it closer to a 1G Model T. Since then, I've used it nearly every week with no problems. So maybe it was due. It did take a tumble off the top of a stack a couple of years ago, but that didn't appear to cause any trouble other than breaking one of the tubes.

The amp hasn't had any caps replaced since the incident in the 70s. Forty-year-old amp, forty-year-old electrolytic caps, that's an obvious place to look. A new set of tubes is never a bad thing to do, whether it solves the problem or not. After the stage dive a couple of years ago, something might have been knocked loose that's only now creating a problem. Guess I'll find out.

When will I get to it? Who knows? I tend to procrastinate pretty badly. I have other, higher priorities this weekend, and once something slips down my list, it can be hard to get it back on top, but we'll see. I'll keep you posted.
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